1. Field of the Invention
The present invention relates to micromachines and more particularly to devices for electrically levitating the moveable elements of a micromechanical structure.
2. Description of the Prior Art
In the field of microfabrication, it has been the general practice to employ many of the principles previously used in the microchip technology to microengineer tiny mechanical objects designed to act as machines. Such miniature machines often include a substrate made of a silicon wafer, or the like, on which are mounted moveable microstructures in the form of diaphragms, springs, sliders, cantilever beams, gears, levers, and driven motors. These elements often have dimensions in the order of one-tenth to one-thousand micrometers in any linear direction.
Although such micromachines have been successfully fabricated and tested, they have not proved entirely satisfactory under all conditions of service for the reason that considerable difficulty has been experienced in fabricating satisfactory bearings for those machine elements that have moving parts. The problem of friction and wear due to mechanical contact between stationary and moving parts in microdevices that produce motion is one of paramount concern because the performance and reliability of these miniature devices will depend critically on the amount of friction generated.
More specifically, U.S. Pat. No. 4,740,410 discloses a method that employs thin-film technology for making micromechanical elements having moveable parts which undergo rotational motion, rectilinear translation or a combination thereof. The patent suggests that micromechanical joints made in accordance with the disclosed method may be used to create such micromachines as optical reflectors or shutters, valves for fluids, ratchets, timing elements, analog computing elements, digital logic elements, accelerometers, engine-knock sensors, and force or torque transducers. The structures disclosed in U.S. Pat. No. 4,740,410 show frictional microjoints that have contacting surfaces of considerable size which can seriously effect the operation and sensitivity of the final device. Those concerned with the development of micromachines have recognized the need for reducing or eliminating the frictional wear created by microjoints of the type disclosed in this patent. For example, Gannon in "Micromachine Magic", Popular Science, March 1989, pp. 88-92 discusses the concern that workers in the field have expressed about friction in such micromachines. In this publication, Gannon quotes one such worker in the field as follows: "The friction issues are probably the most unknown, probably the most perplexing and potentially the most devastating".
Various methods have been proposed to reduce the shearing forces created by contact friction in such devices. These include the use of "bushings" that reduce the contact area and the use of edge-charged contacts that can provide additional sideward pull on the moving components to reduce the normal forces at the contacting surfaces. To date, most of the proposed designs still entail some mechanical contact between the moving and the stationary parts. Clearly, there is a critical need for the development of microstructures that can mitigate or even eliminate the problems associated with the friction encountered between moving parts in a micromachine.
The use of magnetic and electrostatic bearings to reduce or eliminate friction by levitating moveable machine components is well known. U.S. Pat. No. 4,761,579 describes levitation devices that can be used to levitate machine parts by the use of alternating currents passing through conducting wires that act as electro-magnets. These structures are inherently massive and, as such, are primarily meant to be used for supporting large objects. Electrostatic levitation systems are described in the following U.S. Pat. Nos.: 4,521,854; 3,482,455; 3,847,026; and 4,074,580. Again, these inventions are designed to be used in relatively large scale systems. In general, these levitation devices are complex and often require sophisticated feedback circuits that continuously monitor the position of a moving component to provide a feedback signal that modulates a voltage so as to maintain levitation. As a result, these inventions require the use of multiple components of substantial size that are not suitable for use in micromachines.
In U.S. Pat. No. 4,780,331 a method and an apparatus are described for electrostatically charging the minute particles of a fine powder so as to cause the powder particles to levitate and move toward a charged workpiece. In this invention, the powder particles move randomly toward the workpiece under the bias of an electrical field. The principles of this invention are not suitable for application to the fabrication of a micromachine wherein there are moving parts that must be stabilized at a fixed location or must move over a predefined path to perform some predefined function.
While the principles of levitation have been used with much success in reducing friction between moving parts of many machines, there has been no practical levitation system developed that can reasonably be applied to small scale structures such as micromachines. It is an object of the present invention to provide a levitation system for use with small scale structures.